Signals from radio access devices that operate on Institute of Electrical and Electronics Engineers (IEEE) 802.11, IEEE 802.15.4, Bluetooth technology or other comparable standards, generate some amount of energy outside of their approved spectrum band, which may be referred to as side band emissions.
Although devices that transmit/receive such signals employ filtering to minimize RF interference from adjacent channels, this interference also generates side lobe energy that falls into the pass band of signals that operate on IEEE 802.11, IEEE 802.15.4, Bluetooth technology or other comparable standards. If the adjacent channel interference (ACI) is much stronger than the 802.11 signal, side band energy from the ACI may dominate the channel's noise floor (NF) (e.g., increase the effective NF of the frequency channel).
The high level of ACI results in a NF that dominates the 802.11 channel's Signal to Interference and Noise Ratio (SINR), which in turn reduces WLAN signal strength by causing, for example, processing artifacts and quantization errors. Increased noise floor due to adjacent channel interference decreases the receive sensitivity of a device yielding unacceptably low throughput, and sometimes, even no throughput at all. Furthermore, in case of high ACI, the device will not be able to take any share of the airtime, e.g., it will not have access to the medium, since the interfering signals numb the Device's receiver (e.g., render the device's receiver inoperable with respect to receiving and filtering signals).
FIG. 1 illustrates the effect of adjacent interfering channel on a receiving channel's noise floor. As illustrated in FIG. 1, the energy emission from the adjacent channel increases the NF of the receive channel from the Noise Floor (dashed line) to the Effective Noise Floor (solid line), thus resulting in a lower SINR 2, as opposed to the SINR 1, if no interference was received at the receive channel from the adjacent channel.
IEEE specifies the required minimum spectral mask for the transmitted signals. However, even though transmitted signals satisfy the spectral mask requirements, the transmitters may still emit unwanted signals at image frequencies, which also constitute a source of interference for other nodes in the vicinity of a receiver at a given node. In order to suppress unwanted signals, each device applies adjacent channel rejection (ACR) at its receiver. ACR performance is inherently related to RF hardware design, such as the design of sideband and image filters. For example, two Access Point devices (APs), both satisfying the IEEE criterion on spectral requirements, in close vicinity of each other, may numb one another's receiver if the frequencies they operate at, cause high adjacent channel interference to one another. This effect may be one sided, meaning that one AP numbs the receiver of the other AP, yielding the numbed AP to stop all communications, while the other AP takes full share of the medium, i.e., airtime.